How big should the screen be?

It is important to choose a screen size based on the viewing experience rather than the size of your wall. A well chosen screen will allow the audience to comfortably view the picture without being either overwhelmed or straining their eyes.

Home Theatre and Movies

For a home theatre the audience should be able to see the whole screen comfortably without having to "look left and right". Measure the closest viewing
distance and divide by two, to obtain the recommended maximum width for a home theatre screen. At home it is better to err on the side of a smaller rather
than larger screen.

Business Presentations

For a business presentation it's important for your audience to be able to read and comprehend what is on the screen. In an auditorium, hall or other large viewing area, measure the furthest viewing distance and divide this by six to find the smallest recommended screen width.
For business presentations it's better to opt for a larger size so the people at the back can comfortably read the screen.

Choosing an aspect ratio

A good general rule of thumb is to match the aspect ratio of your screen to your projector's native output. Often, this will mean 16:9 (HDTV) for home use and 4:3 (VF) at work and in schools. If your projector is fitted with an anamorphic lens you'll need a 2.35:1 screen, available as a custom build.